1. Field of the Invention
Structural stress measurements are critical to aircraft and many other structures. Overstressed members require expensive inspections at best and can fail in flight at worst. Uncertainty about actual loads borne by structural members forces aircraft designers to increase the mass of critical parts as a safety margin. That increased mass reduces the aircraft""s performance while increasing stress on other structural members. Typically static structures such as bridges, dams and buildings must be designed for external loads imparted by earthquake, wind and even sunlight. Severe external loads, such as those imparted by turbulence on an aircraft or earthquake on a bridge, require expensive and time consuming inspections before the machine or structure can be returned to service. The art of determining loads has attempted to reduce that uncertainty with a myriad of external sensors overlaid on structures. Those external sensors are separate, expensive systems which require precision attachments and calibration. None of them is capable of actually measuring internal loads in structural members so each measures an external strain, position or even optical characteristic in order to estimate internal stress.
2. Description of the Related Art
One of the many attempts to measure stress or loads in structures is illustrated in U.S. Pat. No. 5,381,005 by Chazelas and Turpin. Their Optical Fiber Stress Detector Using Amplitude Modulation is a variation on fiber optic detectors which are overlaid on or embedded in structural elements. While functional, such overlaid systems all share the limitations of any system which adds complexity, cost and weight while not contributing to a structure""s function. The present invention avoids most of those problems by using functioning fasteners as sensors where possible.
Optical sensors are common in this art for their precision and non-metallic composition. The present invention""s primary function is determining dynamic loads in aircraft and other structures while in operation. That function is met for ships with the optical sensors described in U.S. Pat. No. 5,942,750 Method and Device for Continuous Monitoring of Dynamic Loads by Sannerhaugen and Hellvik. As is always the case in the art, ""750 teaches an externally applied apparatus of sensors overlaid on the structure. Optical strain sensors are rigidly attached to a ships hull and connected by fiber optic cable to the processing means. The entire cost of this system is added to the structure""s cost while many potential failure points are evident, such as the security of each strain sensor""s attachment.
Another variation on this theme is seen in U.S. Pat. No. 5,569,857 Vehicle Stress Detecting and Measuring Method and Stress Detecting Device Using said Method by Miyazaki. That inventor isolated the shear vector from various forces within a structural member by, again, using external sensors applied with the typical drawbacks.
An example of the extremes reached in attempting to determine stress in structural members is seen in U.S. Pat. No. 5,699,159 Loadmeter Employing Birefringence to Measure Mechanical Loads and Stresses by Mason. His invention is interesting in that loads are calculated without physical contact with the member. In addition to the other problems inherent in applying an external measurement system, it requires the added complexity of a xe2x80x9cbirefringent coatingxe2x80x9d on the member with cameras and computers equipped to evaluate the xe2x80x9cspectral demodulationxe2x80x9d.
Various strain gages are also common in the art. The present invention is capable of calculating the external loads on a member or part through knowledge of the loads or stresses resulting within that part. Moses and Hogan meet the same need using strain gages in U.S. Pat. No. 5,905,212 Load and Deflection Measurement System for Elastomeric Bearings. Once again, conventional devices such as the xe2x80x9cmetal foil strain gagesxe2x80x9d are applied to the outside of the member in order to indirectly determine the relevant forces. Those external sensors require precision application and maintenance while only estimating the internal loads.
There are several interesting variations on the external sensor theme. One objective of the present invention is detecting cracks. That need is addressed by Belk and White in U.S. Pat. No. 5,969,260 Remotely Interrogatable Apparatus and Method for Detecting Defects in Structural Members. As with the rest of the art, the Belk et al invention requires the overlay of a sensing system on the structural member in question; in this case a wire is incorporated in composite structures. While functional, it includes further limits such as one-time operation (since breaking the detection circuit renders it inoperative), no warning before an overload causes the member to fail or crack, and intermittent operation available only when a technician connects more hardware to the detection device. The present invention meets the same need with none of these limitations.
Position sensing is also used to estimate loads. Another objective of the present invention is determining the payload weight and distribution of a vehicle. This objective is also attempted in U.S. Pat. No. 5,973,273 Method for Determining Weight of a Vehicle in Motion by Tal and Elad. Once again, limitations of the art are severe. ""273 teaches measuring the deflection of vehicle axles with conventional, external sensors in order to infer the weight borne by them. That method only works when the vehicle is in motion and only if the characteristics of the springs are know with precision. The present invention would serve the same purpose by measuring the load inside relevant structures at rest or in motion and with no need to know the characteristics of the vehicle""s suspension.
The advantages over all such externally applied conventional sensors are many.
An art previously unrelated to this subject of measuring internal structural stresses is that of setting the torque on a fastener. Safe and reliable joints require fasteners installed to the proper torque. This art includes mechanical devices such as torque wrenches or indicating pins mounted on fasteners which move as torque is applied. More precision is possible through he use of ultrasonic transceivers to measure the time of flight of a sound pulse, i.e. a bolt""s length, as it elongates during tightening. That technique of setting torque on a bolt by measuring elongation with ultrasonics is well known and, while certainly possible with the present invention, is not an object of this invention. An important segment of this unrelated torquing art is the development of fasteners containing piezoelectric crystals permanently embedded within.
The torquing art with the aid of ultrasonics is summarized in Fastener Incorporating Ultrasonic Transducer U.S. Pat. No. 4,294,122 by Couchman. He teaches providing fasteners modified with permanent transducers for actively measuring preloads on those fasteners using ultrasonic signals. His modified fastener incorporates an acoustic transducer permanently anchored in an opening formed in its head. Fasteners such as that described by Couchman ""122 are required for the present invention. His stated objective is, xe2x80x9c. . . to obtain more accurate preloading measurements and also to provide for improved quality control inspection of the fastener and to allow acoustic monitoring of critical fasteners.xe2x80x9d with the sensing tool temporarily pressed against the surface of the fastener. In other words, he developed a method for measuring the preload torque in fasteners and then rechecking that torque on occasion. Here again, the inventor""s sole emphasis is on the binding force exerted by the fastener on the materials which it binds.
In the related patent Fastener Incorporating Removable Ultrasonic Transducer U.S. Pat. No. 4,295,377, Couchman repeats his points from ""122 while incorporating a xe2x80x9ctransducer (which) may be removed for repair or replacement purposes.xe2x80x9d. Improved acoustic contact in order to accurately determine the torque on a fastener is the objective; though his removable transducer would not function in the present invention. Couchman ""377 does take measuring preload one step further with, xe2x80x9c. . . as well as subsequent measurements to detect for flaws or cracks or which may be employed subsequently for monitoring purposes.xe2x80x9d. This inventor makes it clear that his intent is to improve the fastener torquing art with provisions for occasionally rechecking that torque, as well as checking fastener integrity by detecting cracks in said fasteners, while temporarily pressing the xe2x80x9cremovable ultrasonic transducerxe2x80x9d against the individual fastener in question. In both ""122 and ""377, Couchman also provides detailed work on such ultrasonic fasteners themselves and the circuitry required to interrogate them. As with other inventions in the torquing art, these are not concerned with dynamic loads during operation of the structure.
Couchman ""377 states that xe2x80x9cOne may use the embodiments of FIGS. 2-4 (i.e. the bolt) to look for acoustic emissions or for other diagnostic purposes while the vehicle is in operation or flight and which may result from bolt or adjacent structural failure.xe2x80x9d By itself, that sentence seems to imply a function similar to the present invention. But that paragraph continues with, xe2x80x9cIn this embodiment, the pulser 121 will NOT be employed. The output of the transducer will be coupled to circuits 127, 129, and 131 and the output of circuit 131 will be monitored. FIG. 6 illustrates one way in which ACOUSTIC EMISSIONS, which may occur while the vehicle is in operation or in flight due to bolt or adjacent structural failure, may be monitored. The embodiment of FIG. 2 is shown in this figure. The system 93 will have a suitable readout for monitoring for acoustic emissions from the bolt or from the adjacent structure.xe2x80x9d (Capitalization added for emphasis). Couchman is saying that his bolt may be used, with the ultrasonic signal generating system disconnected, as a microphone. If the bolt or surrounding structure breaks and thereby makes noise, his invention can hear it. That in no way implies an ability to figure out how many kilo Newtons of force the wing structure bears which is an example of the present invention""s use.
Well known piezoelectric or ultrasonic fasteners are described in the Couchman patents and also by Kibblewhite who authored a series of patents, culminating in U.S. Pat. No. 5,220,839 Ultrasonic Load Measuring Device with Control Feature, which teach improvements in the art of constructing fasteners incorporating piezoelectric media. Such fasteners are not claimed in this invention and would be purchased on the open market for use both in their anticipated function as precision fasteners and their unanticipated function or new use herein as sensors. While Kibblewhite uses similar terms with statements such as, xe2x80x9cYet another object of the present invention is to provide a method and an apparatus for reliably and removably coupling a load measuring device with a load indicating member or fastener and for repeatedly or continuously monitoring the load indicating member or fastener.xe2x80x9d, his invention is distinct. In context, it is clear that Kibblewhite and Couchman both invented variations on ultrasonic bolts, nothing more. xe2x80x9cContinuousxe2x80x9d to them means that a properly equipped wrench may interrogate their fasteners xe2x80x9ccontinuouslyxe2x80x9d while turning it. The wrench must then be removed in order for the machine or structure containing their fastener to operate. This point is reinforced by the first line of the first claim in a Kibblewhite patent, xe2x80x9cA method of making a load indicating fastener . . . xe2x80x9d.
A critical point, at the heart of the present invention, is that fasteners and other fittings, equipped with transceivers, function in entirely new use as sensors. In this new use, modified fittings are able to measure stress or loads inside the structural members into which they are installed. They perform this new use regardless of whether or not they perform other functions such as securing joints, suspending loads or reinforcing a structure. All previous ultrasonic fastener art has been solely concerned with measuring load within a fastener and does not teach or anticipate determining stress or load in a machine or structure. Even though a fastener may also be defined as a member of a machine or structure, it must be understood that the acoustic fasteners invented by Couchman, Kibblewhite and others are used here as sensors installed in structural members such as aircraft wings. Understood properly, it is obvious that such fasteners are to structural members as thermometers are to the human body. The fact that acoustic fasteners may also form joints speaks to the efficiency of the present invention but is not necessary to the fastener""s function as a sensor.
The most interesting application of ultrasonic measurement technology to fasteners is seen in the recent patent U.S. Pat. No. 5,970,798 Ultrasonic Bolt Gage by Gleman and Rowe for NASA. As with others working to improve the art of torque measurement, they are concerned only with the preload on critical fasteners which must join structural members with the precision and reliability required by the space program. Gleman and Rowe ""798 improves the torquing art by satisfying that preload measurement requirement with a multiplexing arrangement that allows more than one fastener to be interrogated during a test, albeit only one at a time. Their connection and interrogation method would not work for the present invention, however, as Gleman and Rowe ""798 teaches a temporary connection to the fasteners, an ultrasonic transceiver mounted in the test system rather than in the fastener (so as to not lose the transceivers on launch) plus interrogating those fasteners one at a time. The Gleman and Rowe ""798 patent, which discloses a multiplexed system using ultrasonics to determine the torque on multiple fasteners, fails to mention any possibility of permanent attachments at the fasteners; or measuring any load beyond that inside the fastener; or any possibility of determining loads within the structure in operation (during launch for Gleman and Rowe ""798); or any possibility that monitoring the loads at fastener locations could contribute to the safety and operation of the structure. All of those advantages and more are obtained by the present invention.
The present invention works regardless of whether or not the piezoelectric equipped fasteners are also used to form joints. One possible application illustrating that versatility is U.S. Pat. No. 5,970,679 Metal Load bearing Structure having Structural Connections with no Welding or Drilling by Amore. As the title says, this invention creates a loadbearing structure with no fasteners. The present invention allows piezoelectric equipped fasteners to be installed at any relevant location to determine the stresses within such a structure in spit e of no joining function for the fastener.
This invention relates to measuring static or dynamic stress in the load bearing members of aircraft and other structures or machinery. More specifically, it relates to taking such measurements from sensors located directly inside the load bearing member in question rather than inferring the stresses from indirect measurements. The current art estimates such stress through indirect indicators such as strain gages (which measure stress in the gage and infer the stress in the associated member) and accelerometers (which measure acceleration experienced by the meter and infer conditions within the associated machine). This invention also addresses detecting failures in load bearing members, such as cracks and failed fasteners, which must now be detected with manual inspections often requiring purpose built Non-Destructive Inspection hardware and costly down time. The innovative sensors proposed for this new method are fasteners equipped with piezoelectric crystals or other means for ultrasonic measurement. The present invention is applicable regardless of the material comprising the structural member such as metal, composite, wood or plastic.
Currently, unreliable fasteners and fastener torque settings require over-engineering for safety with the associated weight, cost and performance penalties. Imprecise load measurement, or the inability to measure such stresses at all, is another problem that requires over engineeringxe2x80x94the safety factor engineers add to compensate for uncertainty. Expensive and potentially lethal machinery, such as aircraft, is routinely retired based on time or operational cycles because the true fatigue within the machine cannot be known.
In all previous load or stress measurement applications, the focus has always been on adding external sensors such as optical or metal foil strain gages. This present invention is novel in that the focus is now on conditions inside the machine or structure: the stresses experienced by structural members and the performance of the machine itself are the issues rather than an estimation of those forces from the output of external sensors. As distinct from Couchman ""122, ultrasonic principles are used here to directly measure stresses in load bearing members without regard for the torque on the fastener being used as the sensor. In other words, the present invention is concerned with the change in loads during operation as measured by properly equipped fasteners.
In the system and method of this invention, the change in the stress or load experienced by a structural member and the largest absolute magnitude of that stress is important with the preload torque on a fastener being relevant only for comparison. Such changing loads during operation are commonly referred to as xe2x80x9cdynamicxe2x80x9d.
Piezoelectric equipped or xe2x80x9cultrasonicxe2x80x9d fasteners work by transmitting ultrasonic pulses through the fastener from a piezoelectric crystal transducer which is commonly affixed to one end. The time required for the signal to reflect back to said transducer can be analyzed to indicate the fastener""s deformation or elongation which is a direct measure of the stress being exerted on that fastener by the structure. Since the fastener is immovably mounted within the structural member, the loads are the same in the fastener as in the structure. Changes in a signal""s time of flight are a direct measure of changes in the stress experienced by the associated fastener as well as in the associated structure.
The most common application of the present invention would use piezoelectric equipped fasteners in the entirely different application of direct and active measurement of both static and dynamic stresses. In this context, the term stress measurement refers to measuring the load or stress, and changes in the load or stress, within the fasteners which is a direct indication of conditions and the changes in conditions within the surrounding structure into which said fasteners are installed. It is also possible to install piezoelectric transducers in other suitable load bearing media such as the steel rebar rods used in reinforced concrete. The load or stress measuring method would be substantially the same regardless of which load bearing media is used.
Rather than measuring the elongation of fasteners only for setting or checking torque and fastener integrity, this invention would measure stress at critical points in a structure or machine through those same fasteners. Such stress measurements would be made as required. Some applications, such as bridges, would conceivably be measured intermittently and others, such as aircraft or engines, would more likely be monitored constantly. Known ultrasonic stress measurement technology is herein used for the entirely new purpose of measuring stress, not in a fastener, but in the surrounding member of the structure or machine into which the fastener is fixed. Of course, the conventional application of setting and checking torque is still possible.
Ultrasonic fasteners can also be installed purely as sensors with no connective function at all. This technology also facilitates measuring the load on fittings which are not strictly considered fasteners but which are configured so as to allow ultrasonic measurement of their deformation.